Conventional powertrain mounts exist in many varieties and generally operate to provide engine isolation while concurrently controlling engine motion. One common type of mount is the elastomeric mount. The typical damping versus frequency performance curve for an elastomeric mount is relatively constant. This type of "plain" elastomeric mount provides a fairly constant damping rate across the range of frequencies typically encountered to in a given application. The level of damping provided is typically increased or decreased for the application by pre-selecting an elastomeric material having different properties and physical dimensions. Once the mount is constructed, set damping rate characteristics are provided regardless of the actual operating conditions that the mount encounters. Therefore, another general characteristic of a typical plain elastomeric mount, is that it provides a relatively uniform damping rate regardless of vibrational input amplitude, or one that decreases slightly with amplitude. It has been recognized as desirable to provide a mount that exhibits relatively high damping responses at low frequencies and relatively low damping responses at high frequencies. It has also been recognized that a desirable operating characteristic of a mount is to have a high damping coefficient for relatively high amplitude inputs and a relatively low damping coefficient for lower amplitude inputs. In part, to provide these desirable operating characteristics the hydraulic mount was developed.
The typical hydraulic mount includes a pumping chamber surrounded by relatively thick elastomeric walls with an orifice track opening to the chamber and extending to a reservoir that is typically surrounded by a flexible rubber diaphragm. The reservoir is typically located on the opposite side of a partition from the pumping chamber. During compression operation, fluid is pressurized in the pumping chamber and is caused to flow through the orifice track to the reservoir. During rebound operation, fluid is drawn back to the pumping chamber from the reservoir. The geometry of the pumping chamber, orifice track and reservoir are tuned so that the fluid in the orifice track resonates at certain frequencies. This is used to provide a peak damping effect at a selected frequency to reduce vehicle harshness from road induced vibrations. Generally, hydraulic mounts are viewed as preferable in many applications. In comparing the response achieved by the plain elastomeric mount to that achieved by the hydraulic mount, it is known that a hydraulic mount can be utilized to significantly reduce wheel hop induced vibrations, but can also result in an overall response that has the undesirable consequence of a slightly harsher ride characteristic. One hindrance in applying the hydraulic mount to certain vehicles is that in order to adequately control engine motion during torque transients, they don't provide a sufficiently soft characteristic to attenuate harsh running engine vibrations. Accordingly, it would be preferable if the hydraulic mount could be applied to a wider range of applications including those where engine generated vibration isolation must be maximized.
A known manner of achieving additional incremental increases in the performance characteristics of hydraulic mounts at selected frequency ranges, is to add electronic control to the dynamic characteristics of the mount to provide a preprogrammed active ability to change the response of the mount to optimize damping of the encountered vibrations. In a known type of electronically controlled mount, a relatively powerful solenoid actuator is provided to vary an orifice, effecting fluid flow control between the pumping chamber and reservoir of the mount. While this solution to the aforementioned drawbacks is relatively successful, it tends to be rather costly and typically requires a mount of relatively large size. This increased size and complexity leads to significant restrictions in the number of applications within which the controlled mount can be utilized. Accordingly, a new type of controllable mount is required that is relatively simple, while having the capability to provide soft operating characteristics in combination with hard operating characteristics for torque transient conditions.